1. Technical Field
This invention relates to coolable airfoils of the type used in high temperature rotary machines such as gas turbine engines and more particularly, to an improved scheme for cooling the leading edge portion of such airfoils, with enhanced efficiency.
2. Background Art
Gas turbine engines for powering aircraft, watercraft or for industrial use include a compressor, a combustor and a turbine. Air drawn into the engine is compressed in the compressor causing the temperature and pressure of the air to rise. The compressed air is mixed with fuel in the combustor and the mixture is burned. The products of combustion are expanded in the turbine section to provide useful work in powering the turbine (and in turn the compressor to which the turbine is mechanically connected) as well as thrust.
The turbine typically comprises a series of rotatable disks having a plurality of blades extending radially outwardly therefrom. The blades receive energy from the hot products of combustion and in reaction thereto, cause the disks to which they are attached, to rotate. The products of combustion at temperatures in the range of 2500.degree. F.-3000.degree. F., are capable of damaging the turbine blades unless the blades are cooled interiorly thereof with the cooling fluid such as air bled from the compressor.
As is well known, modem turbine blades are provided with internal passages through which cooling air is channeled and then expelled, typically along the airfoil surfaces of the blades. Typical of the cooling passage arrangements employed in modern turbine blades is that found in U.S. Pat. No. 5,403,159 to Green et al. entitled "Coolable Airfoil Structure" and assigned to the assignee of the present invention. Perhaps the most difficult portions of turbine blades to cool are the leading edges thereof, particularly the leading edges of the high pressure turbine blades which encounter the products of combustion directly from the combustor. Prior schemes to cool the interior of turbine blades at the leading edges thereof involved channeling cooling air radially outwardly through the blades, immediately adjacent to an inner surface of the leading edge of the blade. Subsequently, it was learned that a more effective cooling of the blade leading edges could be achieved by impinging cooling air against the inner surface of the leading edge of the blade from an adjacent cooling passage. Although effective, such a cooling scheme does not necessarily make the most efficient use of the cooling air, which, as set forth hereinabove, is typically bled from the compressor of the engine. It will be appreciated that the more air bled off of the compressor for cooling, the less air is available to support combustion, thereby adversely affecting the efficiency of the engine. Optimizing the efficiency of turbine blade, leading edge cooling schemes optimizes the efficiency of the engine into and therefore reduces the amount of fuel burned by the engine, thus enhancing the economy of operation of the engine.